Stepping motors generally have either magnetically permeable rotors or permanent magnet motors. Coils are usually mounted with their axes radially oriented around the rotor and fixed to the stator of the stepping motor. Customarily, several coils spaced about the stator (representing a single motor phase) are interconnected and energized simultaneously in order to position the rotor in a predetermined angular relationship with respect to these coils.
In a variable-reluctance-rotor stepping motor, torque is developed by the rotor until the rotor is positioned so as to minimize the air gap between the rotor and the stator core adjacent the energized coils. In a permanent magnet motor, the rotor moves so as to place a permanent magnet pole as close as possible to the opposite pole generated electromagnetically by an appropriate coil. Many sets (one set for each phase) of these coils can be positioned around the stator with each set energized in succession in order to advance the stepping motor by small angular increments or steps as one set of coils is de-energized and another set of coils is energized.
If a stepping motor is advanced very slowly -- for example, at one step per second -- the rotor velocity goes from a stopped condition to a peak velocity and back to a stopped condition for each advance step. As the speed of the steps is increased, a condition is reached at which the motor never comes to a stop but merely speeds up and slows down at each step interval. As the stepping speed is further increased, the difference between the peak speed and minimum speed of the rotor is reduced until the rotor is going just about as fast as it can go. This is called "slew" speed of the stepping motor. At this slew speed, the angular velocity of the rotor is about as smooth and constant as it will be. The stepping motor then behaves much like a synchronous AC motor. The magnitude of the slew speed depends upon many motor, drive, and system parameters including load characteristics, coil inductance, drive power, and drive circuitry.
In a printing device of the type disclosed in U.S. Pat. No. 3,982,622 granted on Sept. 28, 1976, to J. A. Bellino et al. a print head is advanced from left to right across a record medium or paper. The print head prints a column of dots at each column location on the paper, and the print head can be stopped at any point in its travel across the paper. It must then be capable of reaching printing velocity within one step and maintain printing velocity with a minimum of overshoot, or other velocity error.
It is an object of the present invention to strongly accelerate a stepping motor from rest.
It is another object of the present invention to run a stepping motor at its slew speed after acceleration, but with a minimum of speed error.
It is still another object of the present invention to control the speed of a stepping motor.
It is yet another object of the present invention to control the power supplied to a stepping motor.